1. Advanced LIGO Photodiodes ______
David Jackrel*, Zhilong Rao, James S. Harris
*Dept. of Materials Science and Engineering
Dept. of Electrical Engineering
LSC Meeting – LHO
August 19th, 2004
LIGO-G Z

2. Optoelectronic Device Results
Outline
Introduction
AdLIGO Photodiode Specifications
Device Materials & Structures
Optoelectronic Device Results
I-V Character
Quantum Efficiency
AdLIGO Devices

3. Advanced LIGO Schematic
Auxiliary Length Sensing
Power Stabilization

4. Photodiode Specifications
LIGO I
Advanced LIGO
Detector
Bank of 6PDs
Power Stabilization
Aux. Length (RF) Detection
GW Channel
Steady-State “Power”
0.6 W
~ 300 mA
10 – 100 mW
30 mW
Operating Frequency
~29 MHz
100 kHz
200 MHz
Quantum Efficiency
 80% 
> 80%
 90%
(300mA)/(0.868A/W
* 0.90 QE) = 385mW
Resonating Tank Circuit
Trades w/ Sensitivity

5. GaInNAs vs. InGaAs
GaInNAs
25% InGaAs
53% InGaAs
1064nm light  1.13eV

6. Rear-Illuminated PD Advantages
High Power
Linear Response
High Speed
Conventional PD
Adv. LIGO Rear-Illuminated PD

7. Quantum Efficiency – Thick Sub.
Ext. Efficiency
Bias (Volts)
Optical Power (mW)

8. ‘Thick’ and ‘Thinned’ Structures
0.) Thick Substrate –
Mounted w/ Conducting Epoxy
1.) Epoxy (or BCB) Underfill –
Mounted w/ Flip-Chip Bonder
2.) Photoresist Underfill –
Mounted w/ Flip-Chip Bonder
3.) Photoresist Encapsulant –
Mounted w/ Conducting Epoxy

9. Dark Current: Various Encapsulants
Large Variation Between Devices
 Test Structures, Not the Best Starting Material…
Epoxy
Underfill
Photoresist
Underfill
Photoresist
Encapsulant
BCB
Underfill

10. Thinned Device Quantum Efficiency
 ~ 85%
 ~ 60%

11. AdLIGO Devices Detector Power Stabilization RF Detection GW Channel
Diameter
3 - 5 mm
1.5 mm
1 mm
(or larger?)
Steady-State Power
400 mW
100 mW
50 mW
3-dB 1/RC Bandwidth
1 - 3 MHz
30 MHz
( 180 MHz)
60 MHz
Quantum Efficiency 
> 80 %
> 90 %
Damage Threshold ?
Important?!

12. AdLIGO Devices: Commercial Vendors

13. MBE Crystal Growth Effusion cells for In, Ga, Al Cracking cell for As
N Plasma Source
Effusion cells for In, Ga, Al
Cracking cell for As
Abrupt interfaces
Chamber is under UHV conditions to avoid incorporating contaminants
RHEED can be used to analyze crystal growth in situ due to UHV environment
T= C
Atomic source of nitrogen needed
 Plasma Source!

14. InGaAs vs. GaInNAs PD Designs
2 m
GaInNAs lattice-matched to GaAs!

15. Full Structure Simulated by ATLAS

16. Heterojunction Band Gap Diagram
InAlAs and GaAs transparent at 1.064m
 Absorption occurs in I-region (in  E-field ) p- i- n-
N-layer:
In.25Al.75As or GaAs
Eg2= eV
I-layer:
In.25Ga.75As, or Ga.88In.12N.01As.99
Eg1=1.1eV
P-layer:
In.25Al.75As or GaAs
Eg2= eV

17. Fabrication Procedure
1. P-Contact
Lift-Off Process
2. Mesa Etch
H2SO4:H2O2:H2O (1:1:20)
Etch Rate ~ 0.75 micron/min
3. Au on Heat Sink
Lift-Off Process
4. In Bumps
Lift-Off Process
~3um In Au
GaAs Substrate
PIN Epi- Layers
AlN Heat Sink
In Bumps
PR Encapsulant
AR Coating
Epoxy

18. Standard Epoxy Procedure
Now you can’t pattern the top contact (or the ARC) w/ standard processing...
But, shadow masks might work...
5. Flip-Chip Bond & Under-fill w/ Epoxy
Low viscosity – Fills all of the voids
Makes chip mechanically stable
6. Thin GaAs Substrate
Piranha - H2SO4:H2O2:H2O (1:8:1)
Doesn’t affect epoxy!

19. Shadow Masking

20. Photoresist Underfill Technique
5. Flip-Chip Bond
1 g / 100 sq. micron, 140˚C
6A. Apply Photoresist
SPR 3612 Positive PR
6B. Blanket Exposure
Long exposure, Hard Bake  240˚C

21. Photoresist Underfill Technique (2)
7. Thin GaAs Substrate
Piranha - H2SO4:H2O2:H2O (1:8:1)
Etch Rate ~10 microns / min
8. N-Contact
Lift-off Process
9. Evaporate AR Coating
Etch Process
(SiNx, HfO, …depends where done)

22. Dimensions
6-18 mm
500um
1-4 mm
4-5um
1-2um
3000A
~25 mm

23. Dark Current: Various Encapsulants
Epoxy
Underfill
Photoresist
Underfill
Photoresist
Encapsulant
BCB
Underfill

24. Free-Carrier Absorption
S-I

25. Free-Carrier Absorption

26. Free-Carrier Absorption

27. Free-Carrier Absorption
A = 1 – exp(-tsub•fc) , fc = Nd * 3e-18

28. Thinned Device Photocurrent

29. Thinned Device QE

30. Surface Passivation Results (2)

31. (NH4)2S+ Surface States
GaAs(111)A
GaAs(111)B
(Green and Spicer, 1993)

32. Surface Passivation Results

33. Large InGaAs Devices, –20V Bias

34. InGaAs vs. GaInNAs Dark Current

35. InGaAs vs. GaInNAs Dark Current

36. GaInNAs Dark Current

37. InGaAs Dark Current

38. GaInNAs H2SO4 vs. (NH4)2S+

39. GaInNAs H2SO4 vs. (NH4)2S+

40. Theoretical Saturation Powers

41. Theoretical Saturation Powers

42. RC-Circuit Bode Plot
3-dB
30MHz

43. RC- and LCR- Transmittance
RC-Circuit
LCR-Circuit

44. LCR- Circuit Impedance